The present invention relates to a process for preparing pre-expanded particles of a thermoplastic resin, and more particularly to a process for preparing thermoplastic resin pre-expanded particles, which are suitable for example as a raw material for the production of in-mold foamed articles, without causing environmental pollution.
It is well known to prepare thermoplastic resin pre-expanded particles by a process wherein thermoplastic resin particles are dispersed into an aqueous dispersion medium in a pressure vessel and, after raising the temperature and pressure, are released into a lower pressure zone as disclosed, for example, in Japanese Patent Publication Kokai No. 59-176336.
As one of such processes, for example, Japanese Patent Publication Kokoku No. 7-5784 discloses a process wherein thermoplastic resin particles containing a volatile blowing agent are dispersed into water in a pressure vessel, and after the temperature and pressure in the vessel are elevated to a high temperature and a high pressure, the resulting aqueous dispersion is released into a lower pressure atmosphere which is lower than the pressure in the vessel and into which hot water of at least 40xc2x0 C. is sprayed or steam is introduced, thereby expanding the particles (the process being hereinafter referred to as xe2x80x9cpressure release expansion methodxe2x80x9d). Such a process provides thermoplastic resin pre-expanded particles suitable for the production of cellular molded articles having a small shrinkage by an in-mold foaming method.
In this process, a slightly water-soluble inorganic compound (hereinafter referred to as xe2x80x9cinorganic dispersantxe2x80x9d) and a surface active agent (hereinafter referred to as xe2x80x9cdispersing assistantxe2x80x9d) are used as a dispersant in order to prevent the particles from melt-adhering together under high temperature and high pressure. Even after the aqueous dispersion is released into a lower pressure atmosphere from the vessel under high temperature and high pressure conditions to produce pre-expanded particles, a part of the dispersant still sticks to and remains on the surface of the produced pre-expanded particles (the dispersant sticking and remaining on the surface of pre-expanded particles being also hereinafter referred to as xe2x80x9csticking dispersantxe2x80x9d).
In particular, in case of using a volatile blowing agent such as butane, agglomeration of dispersoid is easy to occur since it has a large effect of plasticizing the surface of the resin particles. In that case, a large amount of an inorganic dispersant and/or a dispersing assistant is required to prevent the agglomeration, and inevitably the amount of the sticking dispersant on the surface of pre-expanded particles tends to increase. If the amount of sticking dispersant is large, the sticking dispersant hinders the pre-expanded particles from well fusing together when the pre-expanded particles are filled in a mold and heated with steam or the like to give in-mold foamed articles.
In order to solve such a problem, after expanding the resin particles, the obtained pre-expanded particles are generally washed with (a) a large amount of water or hot water or (b) an aqueous solution of an acid. These washing liquids containing the dispersant are combined with an aqueous dispersion medium separated from the pre-expanded particles in the expansion step of resin particles and are discharged together as a waste water without recovering them.
However, the washing means (a) requires the use of a large amount of wash water, since the washing effect is small and the efficiency of removing the sticking dispersant is low, thus economically disadvantageous. The washing with hot water also has the problem of increase in utility cost, since a large amount of energy is required in heating water.
On the other hand, the washing means (b) has the problems that equipment cost increases since the equipment must be made resistant to acid, and no effect is obtained if the dispersant used is insoluble in acid.
The washing waste water and the aqueous dispersion medium separated from the expansion step are discharged after being subjected to an appropriate treatment. In case of the washing means (a), a large amount of waste water must be treated before discharge, thus resulting in increase of cost. In case of the washing means (b), since the inorganic dispersant has been solubilized, an additional step such as reprecipitation step or adsorption removal step is required, so not only the process becomes complicated but also the cost increases.
From the viewpoint of environmental protection, it is demanded to decrease the amount of waste water.
It is an object of the present invention to provide a process for preparing pre-expanded particles of thermoplastic resins without causing environmental pollution.
A further object of the present invention is to provide a process for preparing pre-expanded particles of thermoplastic resins according to which a dispersant sticking onto the pre-expanded particles can be efficiently rinsed and removed and the amount of waste water to be treated before the disposal can be remarkably decreased.
A still further object of the present invention is to provide a process for preparing pre-expanded particles of thermoplastic resins which does not require the use of an acid-resistant equipment in the washing step.
These and other objects of the present invention will become apparent from the description hereinafter.
It has now been found that so long as an aqueous dispersion of resin particles to be released from a pressure vessel into a lower pressure atmosphere is stable, the amount of a dispersant to be charged into the aqueous dispersion can be decreased to less than that used in a conventional process using a volatile blowing agent, and the aqueous dispersion is stable even if the amount of a dispersant is decreased when water included in the resin particles is utilized as a blowing agent without impregnating the resin particles with a volatile blowing agent such as butane, or when the expansion of resin particles is carried out by means of releasing the aqueous dispersion kept under a high pressure into a lower pressure atmosphere without using a volatile blowing agent, so the amount of a dispersant sticking to the surface of produced pre-expanded particles can be decreased even if the pre-expanded particles are not washed. It has been also found that the thus prepared pre-expanded particles can be well fused together in a mold to give in-mold foamed articles without being hindered by the remaining dispersant and, as a result, it is not necessary to wash the produced pre-expanded particles with water or an acid and, further, the aqueous dispersion medium can be repeatedly used since a stable aqueous dispersion can be formed even if the dispersion medium containing an inorganic dispersant and a dispersing assistant is recovered and recycled to the pressure vessel without any treatment.
In accordance with the present invention, there is provided a process for preparing pre-expanded particles of a thermoplastic resin which comprises charging particles of a thermoplastic resin and an aqueous dispersion medium containing, as a dispersant, a slightly water-soluble inorganic compound and a surface active agent into a closed pressure vessel, heating the resulting aqueous dispersion to a temperature of not less than the softening point of said thermoplastic resin, releasing said aqueous dispersion from said vessel into an atmosphere of lower pressure than the inner pressure of said vessel, thereby expanding said particles, recovering said aqueous dispersion medium containing the slightly water-soluble inorganic compound and the surface active agent, and recycling it as a dispersion medium to said closed pressure vessel.
The thermoplastic resin particles are preferably polyolefin resin particles, more preferably particles made of a polyolefin resin composition containing a polyolefin resin and 0.01 to 20 parts by weight of a hydrophilic polymer per 100 parts by weight of the polyolefin resin. An ethylene-based ionomer resin is preferred as the hydrophilic polymer.
Preferably, the slightly water-soluble inorganic compound is used in an amount of 0.15 to 2.0 parts by weight per 100 parts by weight of the thermoplastic resin particles.
In the process of the present invention, prior to releasing the aqueous dispersion into a lower pressure atmosphere the inner pressure of the vessel may be raised by introducing an inorganic gas such as nitrogen gas or air to the vessel and subsequently the aqueous dispersion is released while maintaining that pressure, whereby the expansion ratio of the obtained pre-expanded particles can be increased.